Oil Tank Cap Seal

Mart UK said:

The breather is a non-standard routing along the left swingarm. IDK if it's a red valve arrangement.

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Is that from the oil tank (breather) or the engine?

Mart UK said:

Oil out the back right, up to the oil tank cap.

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The return tube should be crimped shut and brazed (bronze welded?) inside the filler neck. The oil should vent horizontally from two holes about 2 inches below the rim of the filler neck.

Yes, that's the arrangement I have, with a breather hose also coming off the top front of the oil tank, around and down to the back left swingarm. The swingarm end of that hose has a metal piece, like a thin speedo drive, sticking out. Is that a valve? It does vent, when I first got the bike I wiped a white emulsion off the end. Not much. Just from standing for a while, I expect.

So I don't think the tank is being pressurized. I'll get a new cork cap seal and see how I get on from there. Thanks.

Mart UK said:

The swingarm end of that hose has a metal piece, like a thin speedo drive, sticking out. Is that a valve?

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Sounds like it might be but without pictures, I think you are probably best placed to answer that question.

Mart UK said:

So I don't think the tank is being pressurized.

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Start the engine, unlatch the cap so it is resting on the filler neck and see what happens.

Could that be the chain oiler?

Hortons Norton said:

Could that be the chain oiler?

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The Mk3 doesn't have the chain oiler.

If the 'separator' gubbins has gone with the OE airbox, may well be either a home made chain oiler, or possibly a 'duck bill' valve???

L.A.B. said:

Is that from the oil tank (breather) or the engine?



The return tube should be crimped shut and brazed (bronze welded?) inside the filler neck. The oil should vent horizontally from two holes about 2 inches below the rim of the filler neck.

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In LAB's photo above the hose just visible at the bottom of the photo is the return from the engine breather, which comes from the back of the timing chest ('73-'75 Commandos). This is often confused and we get questions about correct plumbing all the time by folks when fitting our reed breathers. This breather return hose can pump some oil mist with velocity, so if the tank cap isn't sealing it can allow leaks. Out of the shot of the photo, and further forward on the oil tank, is the atmosphere breather outlet - a pipe that comes out of a raised hump on the top of the tank. This vent hose allows pressure from the oil tank to atmosphere, and I expect is the hose running down along the swingarm (and probably out the back of the bike, beyond the rear tire).

-Kenny

You can tighten down fiberglass reinforced silicon and it doesn't squish out over time like regular silicone - seals better than cork.

Holmeslice said:

In LAB's photo above the hose just visible at the bottom of the photo is the return from the engine breather, which comes from the back of the timing chest ('73-'75 Commandos). This is often confused and we get questions about correct plumbing all the time by folks when fitting our reed breathers. This breather return hose can pump some oil mist with velocity, so if the tank cap isn't sealing it can allow leaks. Out of the shot of the photo, and further forward on the oil tank, is the atmosphere breather outlet - a pipe that comes out of a raised hump on the top of the tank. This vent hose allows pressure from the oil tank to atmosphere, and I expect is the hose running down along the swingarm (and probably out the back of the bike, beyond the rear tire).

-Kenny

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Yes, that's right. Mine is correctly plumbed. The breather hose, connected to the 'atmosphere breather outlet', vents to the outside rear left of the swingarm, angled away from the tyre and nowhere near the chain.

Still experience leakage out of the oil filler cap. I've tracked all the plumbing into / out of the oil tank and it looks fine. The oil return is as per LAB's photo, earlier. The return is working fine at tickover and mild revs with the cap off to check. It flows out the holes each side. I can blow through the breather pipe (the one out of the top front of the oil tank, down to beside the back wheel). No obstruction and the outlet into the pipe is clear. So, I don't think oil pressure is building up in the tank.

The cork washer on the cap was very compressed, so I've put a new one on. It's from AN, from the Mk3 spares drawing. The rim of the tank filler looks absolutely fine, no dents or dings. It has 'ramps' on opposite sides of the filler's lip, I assume for the cap to tighten onto and pull the cap and cork washer down to form a seal. My cap will go on fine up to the point it gets to these ramps in the lip, but it will not turn any further, so it is not using the ramps. It is on solid at this point, with no up/down movement. However, I am still getting some leaking oil from the cap. I would like to wind it onto the ramps, but there's no way. Any more force and I'd be concerned about damaging the oil tank or mountings.

I can't see any reason for it not to go further round. The tank lip is smooth, as is the cap. No sign of burring or any interference. The cork washer is seated flat in the cap. Is there a trick I'm missing?

Thanks.

Mart UK said:

My cap will go on fine up to the point it gets to these ramps in the lip, but it will not turn any further, so it is not using the ramps. It is on solid at this point, with no up/down movement.

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You said previously...:

Mart UK said:

I push it down slightly, turn to get past the small ramp in the cap, then perhaps a further 1/4 of a turn it comes to a hard stop.

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The fact that you can no longer get the cap onto the ramps must be due to the new cork washer not being as compressed as the old one.

Perhaps unrelated but those tanks will crack at the bottom attachment so if it's dripping from the bottom of the tank it's a place to look. I simply mig welded the crack some years ago and it's holding well.

Of course if you're sure the leak is from the cap area disregard.

How sure are you it is leaking from cap? Has the tank got the connection pipe for a chain oiler (though not used on MK3?), perhaps the block off has failed?
Also, I found a seep happening at the large oil output banjo rear side of tank. One or both the copper sealing washers needed renewal, this following a complete strip down and clean out of my tank.
Also check the hose pipe connections are tight and rubber not failed.

I push down and twist also to tighten it completely.. Just replaced my gasket because of seepage past the cap. Anyway the place I ordered gasket from offered 2 different thicknesses to ensure of snug fit.. 'Real Gaskets Tennessee' was the name of the place, also got the one for the fuel cap. No oil or fuel escaping containment now.... They do the sillycone gaskets for rocker covers also.

For fuel cap, you should also check/replace the bit of rubber hose between the sealing plate and actual cap. Original Ceandass caps have a rivetted plate, some later/knock-offs have a threaded nut holding the plate. The rivet can be drilled to get at the rubber bit beneath....changing/renewing this rubber hose makes a huge difference in the pressure the seal makes against the tank neck.

For those making their own gaskets, what techniques are you using to cut the material in a circle?

Tornado said:

For those making their own gaskets, what techniques are you using to cut the material in a circle?

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Get your self a leather hole punch set. For larger stuff I'll use scrap bearing outers, bushes etc.. and turn a knife edge on the ID

Tornado said:

How sure are you it is leaking from cap? Has the tank got the connection pipe for a chain oiler (though not used on MK3?), perhaps the block off has failed?
Also, I found a seep happening at the large oil output banjo rear side of tank. One or both the copper sealing washers needed renewal, this following a complete strip down and clean out of my tank.
Also check the hose pipe connections are tight and rubber not failed.

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I'm certain it's from the cap. I have forced it as far as it will go, just to the start of that ramp and no leak, but I'd guess, with the vibrations, after a day's ride, it has loosened, because it wasn't on fully, it has slowly unwound itself a little. The leak has gone again today, when I wound it as far as I could, but I know it won't stay there, because I'm not engaging on that ramp.

If it's not a common problem and I'm not doing anything obviously wrong, I'll just keep on top of it for now and hope the cork beds in soon and I can get the cap round further.

L.A.B. said:

You said previously...:



The fact that you can no longer get the cap onto the ramps must be due to the new cork washer not being as compressed as the old one.

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Yes, it must be. Guess I'll hope I can gradually compress it with further running. At least I now know what was causing the oil slick.

@Mark UK

Your tribulations with a seep at the oil filler cap reminds me of mine long ago ....

After much experiment with different gasket materials on the oil filler cap, I finally deduced the oil was coming not from the cap, but from under the "L" shaped mounting bracket spot welded to the oil tank top. This is an issue peculiar to the Atlas and earlier Dommies, but I am wondering if you might have a similar crack where the filler neck is welded into the tank.

You might give that area a careful inspection. A porous weld could produce your gremlin. I hope you get it resolved.

Slick